One of the bottlenecks in system performance is feeding instructions to a processor for execution. Traditionally, performance problems associated with retrieving instructions for execution have been addressed by utilizing various levels of caching. Still, access to system RAM and hard disks poses certain performance limitations with regards to retrieving instructions for execution by the processor.
There are two primary types of microprocessor architectures popular in the industry today. These two types are Complex Instruction Set Computer (CISC) and Reduced Instruction Set Computer (RISC). The most popular of the CISC type microprocessors belong to the x86 family of microprocessors produced by Intel and various other companies today. A popular RISC-type microprocessor is the PowerPC 601 (trademark of IBM Corp.). Due to the nature of CISC architecture, the x86 family of microprocessors utilize a variable-length instruction word. However, the PowerPC 601 and other RISC type microprocessors utilize fixed-length instruction words. As a result, it is commonly observed that RISC-type programs occupy a greater amount of storage space than CISC-type programs. Consequently, RISC-type microprocessors are disadvantaged by the greater amount of instruction words that must be fetched, on average, when compared to a CISC-type microprocessor.